A flyback converter is a typical example of a switch mode converter having a transformer or other inductive element. In the case of a flyback converter, a primary side circuit includes a switch which typically may be a power MOSFET, which switchedly connects input power to a primary winding of the transformer. While the switch is closed, that is to say, during a primary stroke, current flows through the primary winding; conversely, while the switch is open, that is to say during a secondary stroke, the current is commuted to a secondary side winding of the transformer and flows in a secondary side circuit, typically in order to charge a capacitor and supply power to the load. A capacitor is typically included in circuit in order that, during the primary stroke, the converter may continue to supply power to the load, by partially discharging the capacitor. In order to operate correctly, and in particular to prevent the capacitor being discharged by a current flowing back through the secondary winding during the secondary stroke, the secondary side circuit typically requires a diode or other rectifying component placed in series with the capacitor.
A switched mode power converter (SMPC) is shown schematically in FIG. 1, having a primary side 11 and a secondary side 12. In this example the SMPC is a flyback converter 10. A primary side 11 is supplied by input voltage Vin, and comprises a switch 13 connected in series with an inductor which forms a primary side winding of a transformer 14. Transformer 14 has a secondary side winding, across which is connected a output capacitor 17. An output voltage Vout is supplied by the switched mode power converter, and may drive a load 18. A rectifier 16, which typically is a diode (although, as will be described herein below, it may be a switch as shown), is connected in series with the capacitor 17.
Although diodes are generally convenient and inexpensive, and thus suitable for use as the rectifying component 16 in many converters, whilst in the forward conducting state there is a voltage drop across the diode. The power dissipated by this forward voltage drop is lost as heat, and this reduces the efficiency of the converter. Recently it has become increasingly common to replace the diode with an active component, in particular a switch, to provide synchronous rectification. By replacing a diode with an actively controlled switch, typically implemented as a MOSFET, the loss associated with the forward voltage Vf drop in the diode may be replaced by the significantly lower loss due to the on-resistance, Rdson, of the transistor, offering a significant improvement in efficiency.
It will be appreciated, that the timing of the turnoff moment of the synchronous rectifier is important: if it is not switched off in time, the output capacitor will start to discharge; conversely if it is switched off too early, part of the energy delivered to the transformer cannot be properly harvested by the secondary side circuit. Flyback, and other, converters can operate in different modes of operation, including discontinuous mode (DCM), boundary conduction mode (BCM) and continuous conduction mode (CCM). In DCM, the secondary current resulting from the primary or magnetizing current, falls to zero, and there is a gap before the next primary stroke starts. Once the secondary current falls to zero the synchronous rectifier should be switched off. After the synchronous rectifier has been turned off (or the secondary side diode stopped conducting, in the case of passive rectification), the voltage across the transformer winding starts to ring at a frequency determined by sum of the parasitic capacitances at the windings in combination with the magnetizing inductance of the transformer.
For reasons relating to the relative significance of the losses associated with each operation of a power MOSFET switch compared with the continuous ohmic losses in the system associated with RMS currents, higher power converters tend to be relatively more efficient when operated in a continuous conduction mode (CCM).
Designing to ensure a degree of tolerance to temporary over-power situations also favours CCM operation: it may be required that a system can accommodate for instance an overpower of 130% of its design power rating for several minutes. During such intervals efficiency is not that important, but power density of the converter is another requirement that is important. Using a flyback in BCM may be possible in over-power situations, but due to the larger peak currents, the transformer is not allowed to go into saturation, even for short periods. Since the magnetic field is proportional to the current, larger peak currents will cause larger magnetic fields and this in turn requires a transformer have a larger physical size, which is not in line with the requirement for high power density. CCM allows for a lower peak current at the same average current, while the average current determines the power converted. This means that CCM can bring a solution which satisfies not only efficiency requirements, but also power density requirements.
However, in CCM by definition there is still current flowing at the end of the secondary stroke, and as a result a steep slope of the secondary current at the end of the secondary stroke occurs. This occurs because of the rapid change of the voltage across the transformer from the secondary voltage Vout (seen as N×Vout) at the primary side, where N:1 is the primary:secondary turns-ratio of the transformer, to −Vin, where Vin is the supply voltage of the converter. Since the secondary diode or synchronous rectification switch (SR switch) is still conducting at that moment, it cannot be turned off instantly. The result is a steep current change, determined by the leakage inductance of the transformer with a voltage N×Vout+Vin across it. Depending on the reverse recovery time of the diode or SR switch, large negative currents can occur. Such negative currents will cause electromagnetic interference (EMI) and an efficiency reduction, because of (a) energy build up in the leakage inductance that will later dissipate and (b) voltage across the recovering diode in combination with large current during part of the reverse recovery interval. The effect is that the expected efficiency gain is not achieved, because of the additional reverse recovery losses.
Whilst it may not possible to completely eliminate such losses, accurate timing of the switching off moment of a SR switch is important to try to keep the losses to a minimum.
Due to the different slew rates of different transistors, it is not trivial to determine the ideal moment to open the SR switch. A similar problem arises in the control of buck converters, in which it is required to accurately control the timing of switching high side and low side switches. A known solution to that problem is disclosed in U.S. Pat. No. 7,446,513, in which the voltage across the first switch is measured and the timing of the second switch is then adapted in order to get an optimum dead time. In other known solutions, a timing pulse is communicated from the primary side to the secondary side, and this information is used to open the SR switch just before the end of the secondary stroke.
FIG. 2 shows a schematic of the SMPC 10 shown in FIG. 1, but including a control circuit 15 which both controls the primary side switch 13, and uses the timing information from the primary side switch to control the SR switch 16. However, it may be shown that for a typical switch mode power converter, timing accuracies of ±25 ns are required in order to put this solution into effect. Such accuracy may not be available, or may be only available by use of complex and therefore expensive circuitry, and there thus may be a need for an alternative solution.